"The most dangerous vulnerability is the Print Spooler service impersonation issue," Symantec Security Response official Joshua Talbot writes in a statement regarding the Microsoft security updates. "This vulnerability has been identified by Symantec as one of the attack vectors built into the notorious Stuxnet threat, which targets industrial control systems. This is evidence the vulnerability is already being exploited in the wild."

The Stuxnet worm was discovered in July and was designed to steal industrial secrets, claiming Siemens as one of its publicly disclosed victims.

Microsoft patched a vulnerability related to Stuxnet last month but the print spooler vulnerability represents a new attack vector for the worm. It was reported to Microsoft by Kaspersky Lab and Symantec.

Kaspersky Lab says that it has "identified yet another zero-day vulnerability in Stuxnet's code, this time an Elevation of Privilege (EoP) vulnerability. The worm uses this to get complete control over the affected system. A second EoP vulnerability was identified by Microsoft personnel, and both vulnerabilities will be fixed in a security bulletin in the near future."

That means there are four vulnerabilities related to Stuxnet, and only two have been patched.

"The fact that Stuxnet uses four previously unidentified vulnerabilities makes the worm a real standout among malware," Kaspersky writes. "It's the first time we've come across a threat that contains so many 'surprises'."

The print spooler error fixed Tuesday affects a wide range of software including Windows 7, Windows Vista, Windows Server 2003, and Windows Server 2008. But it was only rated as "critical" for Windows XP.

"When a computer is configured to use a shared printer, a remote or local attacker can use this vulnerability to gain system-level access and add malicious code to any file in Windows' core directory where operating system files are stored," Talbot of Symantec writes. "System and configuration files in this directory often automatically execute. By overwriting one of these files, the attacker supplied code would automatically run instead of the legitimate file, resulting in the machine being completely compromised."

"This vulnerability allows for a great deal of stealth since no user interaction is required for an attacker to exploit it," Talbot continued. "Affected systems run the gamut, but Windows XP is the most vulnerable. An attacker has to be able to send a 'print to file' command as well as other malicious instructions to the machine. XP most readily facilitates this by having a guest account with anonymous access enabled by default."

It's not unlikely that more vulnerabilitles related to Stuxnet are just waiting to be found, says Jason Miller, data and security team manager at Shavlik Technologies.

"The vulnerability could allow remote code execution if a user opens a specially crafted media file or receives specially crafted streaming content from a Web site or any application that delivers Web content," Microsoft said. "An attacker who successfully exploited this vulnerability could gain the same user rights as the local user."

The MPEG-4 security update is rated critical for various editions of Windows 7, Windows XP, Windows Vista and Windows Server.

Microsoft has not yet patched another publicly reported vulnerability, the Windows DLL load hijacking problem that allows exploitation of 41 Microsoft programs. Microsoft has published a tool to block known attacks, however.